Pseudonym one's forgotten, unfortunately (4)
I believe the answer is:
alas
'unfortunately' is the definition.
(I've seen this before)
'pseudonym one's forgotten' is the wordplay.
'pseudonym' becomes 'alias' (I've seen this before).
'one' becomes 'i' (Roman numeral).
'forgotten' is a deletion indicator.
'alias' with 'i' removed is 'ALAS'.
(Other definitions for alas that I've seen before include "Unhappily" , "Proper, respectable" , "More's the pity" , "Oh dear!" , "Too bad, so sad" .)